Tripods are elements comprising a central body with an axial bore engaged by a shaft; three appendages or stems radially project from the body at 120.degree. with respect to each other, and rollers, sliding in guides provided in an element carried by a second shaft, roll over the surfaces of said stems. Independently of the actual stem shapes, the external stem surfaces comprise a cylindrical or spherical portion which is to be ground so as to have an elliptical or circular profile, depending on the kind of coupling. Moreover, in the case of an elliptical profile, the eccentricity may vary in accordance with the requirements of the coupling users. Very narrow tolerances are permitted for both profiles. It will be apparent that for maximum operation flexibility a grinding machine must be able to execute both kinds of profiles and to adapt itself to the different user requirements in case of elliptical profiles.
In commercially available machines of that kind the workhead allows the usual piece rotation and moreover it can be rocked to vary the piece distance from the grinding wheel during rotation when the elliptical profiles are to be executed.
In such known machines, the piece is worked while being held between the centers, with a conventional faceplate transmission system, or on a fixed piece holder. Whatever the profile to be executed, such machines have the drawback of very long working cycles, with considerable dead times in piece positioning, due to the nature of the piece supporting means. In effect, the three tripod stems must be individually worked and, after completion of the working of a stem, an external loader is to be brought into engagement with the piece, the means retaining the piece on the support are to be released, the piece is to be removed from the support, rotated by 120.degree., and located again accurately and fastened onto the support; after that the external loader is to be moved away. All these operations are made while the grinding machine is stopped.
Moreover, as far as the control of the workhead rocking is concerned, the conventional grinding machines use hydraulic systems or mechanical systems with cams operated by electrical or hydraulic motors. The former systems have the drawback that the viscosity of the fluid used varies with temperature, whereby the response time is not constant. The latter systems have the drawback that the translatory movements are slow. Moreover, observing working tolerances is rather difficult in both cases, especially in case of very narrow tolerances.